1. Field of the Invention
The present invention relates to an information reproducing apparatus such as a magneto-optic disc apparatus, in particular, to an information reproducing apparatus using PRML (Partial Response Maximum Likelihood) method and a reproducing method thereof.
2. Description of the Related Art
In an information reproducing apparatus such as a magneto-optic disc apparatus, as a method for decoding a signal reproduced from a record medium, Viterbi decoding method has been widely used. The Viterbi decoding method is a decoding method for decreasing a bit error rate that takes place in decoding a reproduced signal having a white noise.
Next, an outline of the Viterbi decoding method will be described. A plurality of statuses corresponding to a recording method for a record medium are identified beforehand. The maximum likelihood transition among such statuses is selected by a calculating process corresponding to a signal reproduced from a record medium. Such maximum likelihood transition is selected by an ACS (Adding, Comparing, and Selecting circuit) of a Viterbi decoder. The ACS estimates the maximum likelihood status transition from status transitions equal to the number of statuses. A PMU (Path Memory Unit) that has path memories equal to the number of statuses is disposed on the next stage of the ACS. The PMU generates decoded data as a sequence of decoded data values "1" or "0" corresponding to status transitions equal to the number of statuses from which the ACS estimates the maximum likelihood status transition.
The status transitions equal the number of statuses from which the ACS estimates the maximum likelihood status transition match each other. However, when the quality of a reproduced signal is not good, the status transitions may not match. When they do not match, a settled status transition cannot be obtained. Decoded data that path memories of the PMU generate corresponding to the settled status transition does not match and the reliability thereof is low.
Conventionally, since the ACS cannot recognize status transitions from which it estimates the maximum likelihood status transition, a process corresponding to matches/mismatches of decoded data equal to the number of statuses that are output from the PMU is performed. In other words, assuming that decoded data always matches, decoded data that is output from one of path memories is supplied to the next stage. Alternatively, decoded data that is determined as the most adequate decoded data by the rule of majority is selected from decoded data that is output from all the path memories and the selected decoded data is supplied to the next stage.
In the conventional Viterbi decoder, decoded data corresponding to a status transition is generated. However, since status data that represents the status transition itself is not generated. Thus, the maximum likelihood status transition estimated by the ACS is not recognized. Consequently, it is impossible to evaluate the reliability of decoded data and the signal quality of a reproduced signal corresponding to the maximum likelihood status transition estimated by the ACS.
As described above, the reliability of decoded data should be evaluated depending on whether the status transitions equal to the number of statuses from which the ACS actually estimates the maximum likelihood status transition match each other (namely, whether or not a settled status transition is obtained). To perform such an evaluation, status data that represents status transitions should be generated.